Direct reading magnetic field intensity indicating apparatus



5 Sheets-Sheet 1 April 2, 1968 I E. SCAGGS ETAL DIRECT READING MAGNETICFIELD INTENSITY INDICATING APPARATUS Original Filed Dec. 5, 1963 IG.I/IO I6 {I8 2 0 24 SENSING PHASE I LOW PASS DEvIcE AMPLIFIER DETECTORFILTER MULTIPLIER F 5 CONTROLLED OSCILLATOR COUN YER POWER SUPPLYBALANCED BAND PASS XN FROM MIxER FILTER MULTIPLIER VCO 22 L30 341 K36 lOUTPUT TD CARRIER x DIoDE BAND PAss COQNTER 26 OSCILLATOR MULTIPLIERMIXER FILTER SIGNAL FROM PREAMPLIFIER \IS TO vED 6O 22 as 92 L 52 68 647I i]. F 86 60a 82 84 FROM I -24v TIMER Io INVENTORS LEE E. SCAGGSBERNARD D. SMITH ORNEY FROM VCO 22 A ril 2, 1968 E. SCAGGS ETAL3,376,500

DIRECT READING MAGNETIC FIELD INTENSITY INDICATING APPARATUS OriginalFiled Deci. 1963 5 Sheets-Sheet 2 %|24%, 26; r H+24V I20 5 J I36 I38 fTO MULTIPLIER l 24ewETEcToR FROM FILTER 20 I I I I INVENTORS UnitedStates Patent Ofitice 3,3765% Patented Apr. 2, 1968 3,376,500 DIRECTREADING MAGNETIC FIELD INTENSITY INDICATING APPARATUS Lee E. Scaggs andBernard 1). Smith, San Jose, Calif.,

'assignors to Varian Associates, Palo Alto, Calili, a corporation ofCalifornia Continuation of application Ser. No. 328,358, Dec. 5, 1963.This application Apr. 25, 1966, Ser. No. 544,911

6 Claims. (Cl. 324-.5)

This is a continuation of application Ser. No. 328,358 filed Dec. 5,1963, now abandoned.

This invention relates to magnetic field measuring apparatus, andparticularly to precession type magnetometers that alford a directreadout of magnetic field intensity.

The technique of measuring magnetic field strengths by means of theprecession of atom portions possessing the properties of magnetic momentand gyroscopic moment, such as nuclei, is described in US. Patent Re.23,769, issued to Russell H. Varian on Jan. 12, 1954, entitled, Methodand Means for Correlating Nuclear Properties of Atoms and MagneticFields. In a nuclear free precession magnetometer, the nuclei utilizedare portons in a sample of water or kerosene, for example. The sensingdevice which includes the sample is placed in the magnetic field whichone desires to measure, for example, the earths magnetic field, and astrong polarizing magnetic field H is applied to the protons by means ofa coil surrounding the sample material to polarize the proton magneticn10- ments M substantially at right angles to the earths magnetic field.This polarizing magnetic field is applied sufiiciently long to align thenuclear magnetic moments, for example, three seconds, and is suddenlyturned off with the result that the nuclear magnetic moments are leftsubstantially perpendicular, or at least at some substantial anglerelative to the direction of the earths magnetic field, and are free toprecess about the direction of the earths magnetic field at the Larmorfrequency of the nuclei. The frequency of such precession is directlyproportional to the strength of the magnetic field being measured.

In order to derive the intensity of the magnetic field with such amagnetometer, it is merely necessary to measure the precession signalaccurately since H kf, Where H=earths magnetic field in gammas (10-oersted), k=23.4874 (proton gyrornagnetic ratio) and f=frequency ofprecession in cycles per second. From the expression =23.4874f, it canbe seen that each cycle of the precession signal frequency is equal to23.4874 gamma. Therefore, in order to measure the earths magnetic fieldto an accuracy of 1 gamma or less, it is necessary to employ a counterwhich will measure the precession frequency to fractional parts of onecycle. This has been accomplished in the past by using a counter whichis governed by the following expression:

where N is the displayed number, f is the reference frequency of thecounter, f is the precession frequency and n is the preset number ofcycles of the precession frequency to be counted. This number n islimited by the decay period of the precession signal. By examining theexpression it can be seen that the displayed number N cannot be made toread the magnetic field directly in gammas because N is inverselyproportional to 7, whereas the magnetic field is directly proportionalto f. In order to obtain the magnetic field in gammas, the followingcomputanumber (N) displayed on This conversion process requires anexcessive amount of calculating by a trained individual, or expensivecomputer time. If a computer is used, more complex circuitry is requiredto code the N information properly so that the computer can handle it.

In U.S. Patent 3,070,745, there is described one form of protonprecession magnetometer which utilizes a voltage controlled oscillatorthat idles at a multiple of the input signal or precession frequency.The voltage controlled oscillator is of the relaxation type andgenerates a sine Wave pulse signal having a time constant determined bythe resistive and capacitive (RC) network associated therewith, Thepatented magnetometer requires a stabilizing network and a properlydesigned filter to eliminate harmonies and transients that appear duringsignal processing, especially during the frequency multiplicationprocess. It would be desirable to employ a simple and relativelyinexpensive readout system in a proton precession ma netometer.

An object of this invention is to provide a novel and improvedmagnetometer apparatus.

Another object of this invention is to provide a magnetometer thataffords a direct readout of magnetic field intensity with simple andrelatively inexpensive circuitry.

Another object is to provide a magnetometer which realizes a directsignal readout with an improved signalto-noise ratio and with a minimumof transients.

According to this invention, the readout circuit of a magnetometerapparatus comprises a frequency multiplier circuit which is coupled tothe output circuit of a stabilized voltage controlled oscillator. Theoscillator is locked to an incoming precession signal which isproportional to the intensity of the magnetic field being measured, andthe output signal of the voltage controlled oscillator is fed to thefrequency multiplier to obtain a signal which is an exact multiple ofthe precession frequency. The frequency multiplied signal is applied toa digital counter during a predetermined readout interval for conversionto magnetic field intensity. A delay circuit is incorporated in themagnetometer apparatus for gating on the digital counter during suchreadout interval, whereby transient signals do not appear in the readoutsignal. By means of this combination, a direct readout of magnetic fieldintensity is made possible in a simple and inexpensive manner, with aminimum of transients and a relatively high signal-to-noise ratio.

In an embodiment of this invention, the frequency multiplier comprises acarrier signal generator that supplies a carrier signal to a firstsignal channel and simultaneously to a second channel wherein the signalis mixed with the output signal from the voltage controlled oscillator.The mixed signal and the carrier signal are multiplied in theirrespective channels, and the multiplied signals are fed to a mixer thatproduces a difference signal. This difference signal, which is an exactmultiple of the precession frequency, is converted by the digitalcounter to a direct reading of magnetic field intensity.

The invention will be described in greater detail with reference to theaccompanying drawings in which:

FIG. 1 is a block diagram of the inventive readout system of amagnetometer;

FIGS. 2a-b are illustrative wareforms to aid in the explanation of theinvention;

FIG. 3 is a block diagram of the frequency multiplier used in theapparatus of FIG. 1;

FIG. 4 is a schematic circuit diagram of the phase detector and filterused in the apparatus of FIG. 1;

FIG. 5 is a schematic circuit diagram of the voltage controlledoscillator used in the apparatus of FIG. 1;

FIG. 6 is a schematic diagram of the balanced mixer used in theapparatus depicted in FIG. 3;

FIG. 7 is a schematic circuit diagram of the frequency multipliers usedin the apparatus depicted in FIG. 3; and,

FIG. 8 is a schematic circuit diagram of the diode mixer employed in themultiplier of FIG. 3.

With reference to FIGS. 1 and 2, an embodiment of a readout circuit of amagnetometer apparatus in accordance with this invention comprises asensing device 10 that includes a sample of water that is sealed in acontainer which has a coil of wire axially wound about it, such asdescribed in US. Patent 3,004,211. Typically, the coil is constructed soas to carry a D.C. current of about 6 amps. and produce a polarizingmagnetic field of about 100 gauss within the coil. A sequencer 12automatically operates to pulse a relay (not shown) which, in itsoperation position, couples the coil of the sensing device 10 to a powersupply 14, and, in its released position, couples the sensing device 10to the preamplifier 16 and the free precession counter system. Thesequencer 12 typically operates to couple the sensing device 10 to thepower supply 14 for approximately 2 seconds and then to the countersystem for approximately 1 second. During the period of time when thesensing device 10 is connected to the power supply 14, a polarizingmagnetic field is produced in the coil of the device 10 to align themagnetic moments of the protons in the water in the direction of thepolarizing magnetic field, which is at a substantial angle to thedirection of the field to be measured, preferably normal thereto. Ondisconnect of the sensing device 10 from the power supply 14, thepolarizing field quickly decays and leaves the aligned magnetic momentsto precess in the earths magnetic field. The precession magnetic momentinduces an alternating current in the sensing device 10, thisalternating frequency signal being transmitted to the amplifier 16 underthe control of the sequencer 12. A detailed description of the techniqueof measuring magnetic fields by means of precession of atom nuclei orprotons is found in U.S. Patents -Re. 23,769, 3,004,211 and 3,090,002,among others.

The precession signal derived from the sensing device 10 is passedthrough the preamplifier 16, and the amplified signal is fed to a phasedetector 18 coupled in a feedback loop that includes a lowpass filter 20and a voltage controlled oscillator 22. The voltage controlledoscillator 22 generates a sine wave signal and idles at a predeterminedfrequency, which may be near an anticipated signal frequency in therange of 1000-3125 cycles per second for the earths field at or near sealevel. The phase detector 18 compares the nominal frequency of theoscillator 22 to the incoming signal frequency and provides an errorsignal which is passed through the filter 20 to the oscillator 22,thereby locking the oscillator 22 to the frequency of the incomingsignal. The phase of the oscillator output signal is stabilized towithin 3 degrees of the phase of the incoming signal derived from thesensing device -10. The lowpass filter 20 may be time variable or stepvariable after delay, to facilitate fast lock of the oscillator 22 tothe precession frequency f,,. The filter 20 is controlled by thesequencer 12 as described hereinafter with reference to FIG. 4. With theoscillator 22 locked to the incoming signal, which represents a measureof the earths magnetic field, the stable sine wave output from theoscillator 22 is fed to a frequency multiplier 24. The multiplier 24provides a difference signal which is an exact multiple of the voltagecontrolled oscillator 22 frequency. A digital counter 26 converts thedifference signal to a direct reading of magnetic field intensity.

In operation, the duty cycle of the sensing device 10 is for apredetermined interval (I -t three seconds, by

way of example, whereby the polarization signal is applied for a portionof such interval (t t such as 2 seconds, and is then cut off for theremaining time (t -t one second, as illustrated in FIG. cutoff period (t-t a readout signal is obtainedfrom the voltage controlled oscillator22. At the start of the interval (t -t the counter 26 is inoperative andis actuated after the sequencer 12 triggers a delay circuit 28, which inturn energizes the gate of the counter 26 at t, after a delay of about200 milliseconds. At the instant, t the counter 26 begins to count for apredetermined readout interval t -t the frequency signal from themultiplier 24 is counted, and the number displayed is the magnetic fieldintensity in gammas.

The frequency multiplier24 of FIG. 1 is illustrated in FIG. 3, whereinthe output signal from the voltage controlled oscillator 22 is appliedto a concurrently with a carrier signal from an oscillator 32. The uppersideband of designated as f -i-f (f being the carrier signal frequencyand i the precession signal frequency) is passed through a filter 34,while the lowersideband f f is rejected. In a particular embodiment, fmay be 20 kc. per second, Whereas f is in the range of 1000-3125 cyclesper second for the earths field. The upper sideband signal ismultipliedby a multiplier 36 by an integral number N to produce a signal having afrequency N (j -H Atthe same time, the carrier signal is multiplied by asimilar multiplier 38 to obtain a signal of frequency Nf and bothmultiplied signals are fed to a diode mixer 40. The mixed signal isdirected to a filter 42 that passes only the difference signal between N(f -H and Nf which is Nf an integral multiple of the precessionfrequency. This difference signal, which is an exact multiple of theprecession frequency, is passed to the counter 26 for direct conversionto a measure of magnetic field intensity.

The phase detector 18 and .filter 20 of FIG. 1 are set forth inschematic form in FIG. 4. In operation, the signal from the preamplifier16 is passed through a coupling capacitor 44 and drop resistor 46 to thebases of a pair of complementary symmetry switching transistors 48 and50.

The collector circuit of the switching transistors 48 and 50 includeload resistors 54, 54a, and bias and load resistors 56, 56a, which arecoupled to positive and negative sources of potential respectivelythrough drop re-.

sistors 58, 58a. Zener diodes 60, 60a are coupled between the collectorsof the transistors 48 and 50 land the drop resistors 58, 58a and serveas voltage regulators.

When the signal from the preamplifier 16 is positive going, the NPNtransistor 48 becomes conducting and transistor 50 is cutoff.Conversely, when the signal is negative going, transistor 48 is cutoff.The output signal from the switching transistors 48 and 50 is fed to apair of transistors 62 and 64 that constitute a phase comparator. Theemitters of the transistors 62 and 64 are connected to bias resistors66, 66a to receive a suitable potential from the voltage sources, showsas +24 v. and 24 v. respectively. An NPN transistor 68 is connected tothe emitters of the transistor 62 and 64, the collector of transistor 68i being out of phase with its emitter. The transistor 68'. theoscillator 22 through a conreceives a signal from pling capacitor 70 andpasses such signal to the emitters of the phase detecting transistors 62and 64. A limiting diode 71 is coupled between the oscillator 22 andground to square the sine wave generated by the oscilla'tor 22.

If the precession signal from transistors 48 and 50 are phased relativeto the signal from the oscillator 22 that appears at the bases of thetransistors 62 and 64,

then the output of the phase comparator circuit will be zero DC signal.A variable resistor 75 serves to establish the idle center frequency ofthe voltage controlled oscillator. If there is a phase error, thentransistor 62 i 2. During the balanced mixer 30 1 the mixed signal,which may be i the PNP transistor 50 is conducting and 1 will conductfor a greater time than transistor 64, which will give a resultant DCerror signal. This error signal will provide the necessary correction tothe oscillator 22 to lock the oscillator in phase with the precessionsignal.

A lowpass RC filter network 20 with a variable cutolf frequency iscoupled to the output of the phase comparator, or more specifically tothe collector circuit of the transistors 62 and 64. The filter comprisesa capacitor 72, resistors 74 and 76, capacitor 78 and resistor 80. Thefilter network also incorporates an automatic switch relay 82 having atime constant determined by the conduction of a driver transistor 84,which in turn is controlled by the sequencer or polarizer timer (12).

During the polarization interval t t the positive pulse is applied tothe base of the driver transistor 84 through a diode 86 and through avoltage divider consisting of resistors 88 and 90, which provide an RCtime constant in conjunction with a capactior 92. The diode 86 serves toprevent feedback to the polarization timer (12) from the filter circuit20.

The duty cycle of the relay is controlled by the square wavepolarization signal which charges the capacitor 92, and causes thetransistor 84 to conduct thereby energizing relay 82. When thepolarization signal is removed, the capacitor 92 discharges to groundthrough the voltage divider resistors 88 and 90, thereby causing thetransistor 84 to shutoff and relay 82 to open after a time determined byresistors 88 and 90 and capacitor 92. Thus, when the positive pulse isapplied, the driver transistor 34 becomes conducting and causesenergization of the relay coil connected to the collector of thetransistor 84 causing the relay switch 82 to close. During this time,any DC error signal that appears is passed to the voltage controlledoscillator 22 to rapidly lock the frequency and phase thereof to theprecession signal When it appears. When the relay opens at t +delay, theDC error signal cannot change rapidly because filter 20 has a lowercutoff frequency, but will maintain phase lock between the precessionfrequency and oscillator frequency, and in effect yields a very highsignal-to-noise ratio.

In FIG. 5, the voltage controlled oscillator 22 comprises a field effecttransistor 94 which receives a signal from the filter 20. The fieldeffect transistor 94 serves as a gain control for a PNP transistor 96,which supplies capacitive current to the oscillator transistors 98 and100.

Transistor 98 is a high impedance emitter follower, whereas, transistor100 acts as an amplifier. A feedback resistor 102 and capacitor 103 arecoupled in the emitter circuit of the transistors 98 and 100 to providepositive feedback and thereby cause an oscillatory signal. Resistors104, 106 and 108 provide DC bias to the emitter follower transistor 98,whereas resistors 100, 112 and 114 provide bias to the transistor 96. Apair of diodes 116 and 118, located between a pair of couplingcapacitors 120 and 122, limit the output signal from the collectors ofthe transistors 98 and 100, and diodes 124 and 126, similarly limit theamplitude of the oscillatory signal generated by the oscillator.

A quadrature capacitor 128 conupled in parallel with quadraturecapacitor 130 provides a high impedance at the carrier frequency therebyeffecting a 90 phase shift at the base of the transistor 96. The signalfrom the collector of transistor 96 causes the capacitive characteristicof the transistor 100 to vary because the collector of the transistor 96acts like a varying capacitor. The oscillatory signal is amplified by anamplifier transistor 132, which is coupled through a capacitor 134 tothe input circuits of the multiplier 24 and detector 18. An LC paralleltuned network comprising a capacitor 136 and inductance 138 is coupledto the oscillator transistors to enable tuning to a predeterminedfrequency. A pair of Zener diodes 140 and 142 are connected across thesource of potential to provide voltage regulation. Also, a capacitor 144and resistor 146 provide a decoupling network.

In FIG. 6, a balanced mixer such as utilized in FIG. 3 for block 30 isillustrated in schematic form. A first signal of frequency f which maybe the signal from the voltage controlled oscillator 22, is applied tothe primary of transformer 200, while a second signal of frequency f,,,which may be the carrier signal, is applied through a coupling capacitor195, to the NPN transistor 198. The output of transistor 198 is fed tothe center tap of transformer 200. Thus, f is 180 out of phase at thebase of transistor 201 with at the base of transistor 202; and f is inphase at the base of transistor 201 with at the base of transistor 202.The signals f, and f are mixed by transistors 201 and 202, and the upperand lower sidebands ap pear at the output of transformer 206 as f -l-fand f -f However, i is cancelled by the transformer 206 and is thereforesuppressed at the output of transformer 206-. The output of transformer206 is applied to the filter 34, which rejects the lower sideband (f fand any ,1 which may still be present, but passes f +fp.

In FIG. 7, a multiplier which provides a frequency multiplication of 24by way of example, such as utilized in blocks 36 and 38 of FIG. 3 isrepresented. The signal (1) to be multiplied is provided to an amplifier162 through a coupling capacitor 164, and is passed through atransformer 164 having a center tapped secondary 166. The signal ispassed through a full wave rectifier consisting of diodes 168 and 170,which affords a multiplication of twice the frequency (2f). A rectifierdiode 173 and resistor 175 connected in series with the diodes 168 and170 pass only the sharp spikes of the rectified wave signal to the nextmultiplier stage. A second multiplication stage including a class Camplifier 172 has its output coupled to a multiplier circuit whichenables a frequency multiplication of 3, thus producing a totalmultiplication of 6 A resistor 177 and capacitor 179 serve as adecoupling network for the transistor 172, while resistors 181 and 183provide dc dias to the transistor. The multiplier circuit includes aninductive element 174, and a capacitor 176 shunted by a variablecapacitor 178. The capacitors 176 and 178 serve as tuning capacitors forthe inductance 174. A frequency multiplication of three is achieved byadjusting variable capacitor 178 to tune the multiplier circuit to thethird harmonic of the signal at the collector of transistor 172. Themultiplied signal (6)) is passed through a coupling capacitor to anemitter follower stage 182, which includes a feedback capacitor 184 thatraises the input impedance of the transistor 182. A resistor 186 thatraises the input impedance of the transistor 182. A resistor 186 andcapacitor 188 serves as a decoupling network for the collector circuitof transistor 182.

The signal is taken from. the emitter of the transistor 182 and passedthrough stages 190 and 192, similar to the stages including transistors162 and 172 respectively. Each stage 190 and 192 produces a frequencymultiplication of two, thereby providing a total multiplication of theinput frequency (f) to 24). The stage 192 is adjusted to achieve afrequency multiplication of two by setting its variable capacitor totune i-ts multiplier circuit to the second harmonic of the signal at thecollector of its transistor. The multiplied output signal appears as asine wave signal having negligible harmonics and no transients orspurious oscillations.

In FIG. 8, a mixer such as utilized in FIG. 3 for block 40 isillustrated in schematic form. A first signal f of frequency 24(f -l-ffrom multiplied 36 is applied through a coupling capacitor 148 to theNPN transistor 150, while a second signal f of the frequency 24f fromthe multiplier 38 is applied through coupling capacitor 152 to the NPNtransistor 154. An out-put is derived from the collectors of eachtransistor and summed at a junction point 156. The signals are thenmixed by diode 158. The mixed signal is passed to the base of an emitterfollower transistor 160 which provides the mixed signal to the filter42.

The mixed signal consists of 24 24( f -H 24(f,,+f 24f and 24(f +f +24-fFilter 42 rejects all frequency except the difference frequency 24(f-l-f -24f This difference frequency when reduced to its simplest formequals 24f, or 24 times the precession frequency.

The difference between the multiplied signals derived from themultipliers .36 and 38, this difference signal being an exact multipleof the precession frequency, is utilized by the digital counter 26 forobtaining a direct reading in gammas of the magnetic field intensitybeing measured. The counter 26 may be a commercial Hewlett- PackardModel 5512A with the crystal adjusted for the frequency which will yielda direct readout in gammas.

It is known that H=kf where H is magnetic field in gammas, k is theproton gyrornagnetic ratio which is known to be 23.4874. Therefore f forthe earths field at sea level ranges from 1000-3125 cycles per second.If the commercial counter mentioned in the above paragraph is governedby the expression where N is the displayed number, 1 is the frequency tobe counted, 1, is reference crystal oscillator frequency and n is thepreset number of reference oscillator cycles to be counted, then it canbe shown that a direct reading of magnetic field in gammas will bedisplayed by proper setting of the reference crystal oscillatorfrequency i There has been described herein a readout system for aprecession magnetometer wherein a precession frequency signal ismultiplied by a fixed tuned circuit, such multiplication being achievedover a relatively large frequency range Without the necessity ofchanging multiplier or switching channels. A direct reading of magneticfield intensity may be achieved in gammas with an accuracy of :1 gamma.Improved accuracy can be achieved by multiplying the precessionfrequency by a number greater than 24 and by using a more accuratecounter.

We claim:

1. A readout circuit for a proton precession magnetometer comprising:means for sensing a proton precession signal; an oscillator forgenerating a signal having a predetermined center frequency in thevicinity of the frequency of the precessiOn signal coupled to the outputof said sensing means; a phase detector for comparing the frequency andphase of such precession and oscillator signals; means for locking thefrequency and phase of the oscillator signal to that of the precessionsignal; a frequency multiplier coupled to the output of said oscillatorfor multiplying the oscillator signal; a counter coupled to saidfrequency multiplier for converting the multiplied frequency signaldirectly to a reading of magnetic field intensity; and, a sequencer forcontrolling the duty cycle of such sensing means and such counter.

2. A readout circuit for a proton precession magnetometer comprising:means for sensing a proton precession signal; a voltage controlledoscillator for generating a signal having a predetermined centerfrequency in the vicinity of the frequency of the precession signalcoupled to the output of said sensing means; a phase detector forcomparing the frequency and phase of such precession and oscillatorsignals; a feedback loop coupled between said oscillator and phasedetector to lock the frequency and phase of the oscillator signal tothat of the precession signal; a frequency multiplier coupled to theoutput of said oscillator for multiplying the oscillator signal; adigital counter coupled to'said frequency multiplier for converting themultiplied frequency signal directly to a reading of magnetic fieldintensity; a sequencer for controlling the duty cycle of such sensingmeans and such counter; and, a delay circuit coupled between saidsequencer and counter for controlling the start time of said counter.

3. A readout circuit for a proton precession magnetometer as in claim 2,wherein said multiplier, has 2 signal channels, including a carrieroscillator for supplying a carrier frequency signal to both channels;means for mixing the output signal from said voltage controlledoscillator with said carrier signal in one channel; first and secondmeans for multiplying such mixed signal and such carrier signalseparately in each channel; means for mixing the multiplied signals toderive a difference signal; and, means for filtering the differencesignal.

4. A readout circuit for a proton precession magnetometer comprising:means for obtaining a proton precession signal; a voltage controlledoscillator for generating a signal at the frequency of the precessionsignal coupled to said precession signal obtaining means; means forlocking the frequency and phase of said voltage controlled oscillator tothat of the precession signal; frequency multiplying means coupled tosaid voltage controlled oscillator for multiplying the frequency of saidoscillator and means coupled to said frequency multiplying means fordisplaying magnetic field intensity information.

5. A readout circuit according to claim 4 wherein said frequencymultiplying means includes a first mixer and a first multiplier, a firstmixer and said first multiplier, a first filter coupled to said firstmixer, a second mixer coupled to said first multiplier, a secondmultiplier coupled to said first filter and said second mixer and asecond filter coupled to said second mixer.

6. A readout circuit according to claim 5 wherein said,

first and second multipliers comprise fixed tuned circuits.

References Cited UNITED STATES PATENTS 2,738,462 3/1956 Truxel 324-793,058,053 10/1962 Bloom 324-05 3,066,252 11/1962 Drake 324-05 3,070,74512/ 1962 Serson 324-05 3,090,002 5/1963 Allen 324,-05 3,098,197 7/1963Barringer 324-05 3,103,622 9/1963 Millership 324-05 3,204,178 8/1965Brown 324-05 RUDOLPH V. ROLINEC, Primary Examiner. M. J. LYNCH,Assistant Examiner.

carrier oscillator coupled to said i

1. A READOUT CIRCUIT FOR A PROTON PRECESSION MAGNETOMETER COMPRISING:MEANS FOR SENSING A PROTON PRECESSION SIGNAL; AN OSCILLATOR FORGENERATING A SIGNAL HAVING A PREDETERMINED CENTER FREQUENCY IN THEVICINITY OF THE FREQUENCY OF THE PRECESSION SIGNAL COUPLED TO THE OUTPUTOF SAID SENSING MEANS; A PHASE DETECTOR FOR COMPARING THE FREQUENCY ANDPHASE OF SUCH PRECESSION AND OSCILLATOR SIGNALS; MEANS FOR LOCKING THEFREQUENCY AND PHASE OF THE OSCILLATOR SIGNAL TO THAT OF THE PRECESSIONSIGNAL; A FREQUENCY MULTIPLIER COUPLED TO THE OUTPUT OF SAID OSCILLATORFOR MULTIPLYING THE OSCILLATOR SIGNAL; A COUNTER COUPLED TO SAIDFREQUENCY MULTIPLIER FOR CONVERTING THE MULTIPLIED FREQUENCY SIGNALDIRECTLY TO A READING OF MAGNETIC FIELD INTENSITY; AND, A SEQUENCER FORCONTROLLING THE DUTY CYCLE OF SUCH SENSING MEANS AND SUCH COUNTER.